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This patch implements PV send_IPI method to >> >> reduce the number of vmexits. >> >> It won't reduce the number of VM-exits; in fact, it may increase them on CPUs >> that support IPI virtualization. > >Sure, but I wonder if it reduces the vmexits when there's no APICV or >L2 VM. I thought it can reduce the 2 vmexits to 1? Even without APICv, there is just 1 vmexit due to APIC write (xAPIC mode) or MSR write (x2APIC mode). > >> >> With IPI virtualization enabled, *unicast* and physical-addressing IPIs won't >> cause a VM-exit. > >Right. > >> Instead, the microcode posts interrupts directly to the target >> vCPU. The PV version always causes a VM-exit. > >Yes, but it applies to all PV IPI I think. For multi-cast IPIs, a single hypercall (PV IPI) outperforms multiple ICR writes, even when IPI virtualization is enabled. > >> >> >> >> >> Signed-off-by: Jason Wang >> >> Tested-by: Cindy Lu >> > >> >I think a question here is are we able to see performance improvement >> >in any kind of setup? >> >> It may result in a negative performance impact. > >Userspace can check and enable PV IPI for the case where it suits. Yeah, we need to identify the cases. One example may be for TDX guests, using a PV approach (TDVMCALL) can avoid the #VE cost. > >For example, HyperV did something like: > >void __init hv_apic_init(void) >{ > if (ms_hyperv.hints & HV_X64_CLUSTER_IPI_RECOMMENDED) { > pr_info("Hyper-V: Using IPI hypercalls\n"); > /* > * Set the IPI entry points. > */ > orig_apic = *apic; > > apic_update_callback(send_IPI, hv_send_ipi); > apic_update_callback(send_IPI_mask, hv_send_ipi_mask); > apic_update_callback(send_IPI_mask_allbutself, >hv_send_ipi_mask_allbutself); > apic_update_callback(send_IPI_allbutself, >hv_send_ipi_allbutself); > apic_update_callback(send_IPI_all, hv_send_ipi_all); > apic_update_callback(send_IPI_self, hv_send_ipi_self); >} > >send_IPI_mask is there. > >Thanks > >> >> > >> >Thanks >> > >> > >> >